A Change of Heart can be explained as a sudden change of behavior on a specific person whose desires had been stolen via the Metaverse or Collective Unconscious' machinations. Victims of a change of heart will become better people, and, usually, they will be burdened by their crimes, and will confess all of their sins to the public. However, it can be used for a variety of more malicious effects, such as apathy, suicide, or to manipulate them into obsession with a particular someone. This is technically against the law for assault, although it is incredibly difficult to trace or catch its culprits without anyone with former experience dealing with the Metaverse. Therefore, Persona users such as the Phantom Thieves of Hearts are mandatory for police to track down as culprits.
Persona 5 / Royal
Change of Hearts are instigated by the Phantom Thieves of Hearts against rulers of Palaces, untangling their corruption and reforming them. Against minor targets or any target without a personalized Palace, defeating them in the Metaverse and taking their minor Treasure is enough to convince them to return to their real selves. Many also do apologize for their actions and/or genuinely atone and become better people.
However, against major targets with a Palace, their treasure must be stolen. If this happens, the Palace will crumble. This will always involve defeating the target, since they will protect the treasure at all costs. During another day after their Treasure was stolen, the target will confess their crimes to the world, usually on public television. As a result, targets of the Phantom Thieves do not provide any information to the police, making tracking crimes related to such incidents incredibly difficult. A change of heart can also only be performed if the Treasure has exited its Palace of origin. Therefore, if the Palace owner attempts to stop the Phantom Thieves from escaping at all costs, additional confrontations must be made until the target surrenders.
Although the effects of changing their hearts is supposed to make them better people, in reality, more often than not they are converted into apathetic, weeping husks who abandoned the ability to think for themselves. Converting the targets into weeping husks is fundamental for Yaldabaoth's plan for control, and he deliberately sets up the targets that the Phantom Thieves remove so they can all be contained in the Prison of Regression, which is the reason for the apathy of these major targets. Once Yaldabaoth's trickery and deceit is exposed, the Prison of Regression will cease to exist, allowing future targets, such as Takuto Maruki, to genuinely become better people after their treasure is stolen. Although killed months before, it might also be possible for Kunikazu Okumura to revert back to his state before he was taken over by greed, and become a genuinely good parent towards his daughter Haru.
It was later revealed in Royal that Takuto Maruki has a Persona who lets him change hearts without stealing someone's Treasure, albeit only subconsciously at first and only capable of altering their thoughts. He awakened to this power when his fiance Rumi fell into catatonic depression and had a post-traumatic stress episode at the mere mention of Maruki's name. Desperate to help her, Maruki changed her heart by purging her of her trauma, in exchange for the loss of all of her memories with Maruki. Later on, he managed to subconsciously use the same power on Sumire Yoshizawa to relieve her of her trauma by making her believe that she was her deceased sister Kasumi, albeit only subconsciously (Implying that Sumire reminded him of Rumi's unfortunate circumstances.) After he gains control of Mementos following Yaldabaoth's defeat, Maruki's power turns into altering reality entirely, with the goal of erasing all pain from existence.
Persona Q2: New Cinema Labyrinth
While not explicitly mentioned by name in-game, Doe's true purpose is to allow Persona users from three generations across to break Hikari free from her borderline suicidal depression and self-loathing due to a lifetime of bullying and emotional abuse. This involves changing the endings of the four movies Doe broadcasts in the Cinema to a positive moral code that encourages her to live on. Once the final movie has been changed by defeating Doe himself, he reveals himself to be a Cognition of her father and she confesses like any other targets the Phantom Thieves are changed and was completely snapped out from her catatonic depression.
Persona 5 Strikers
Via Jails, the Change of Hearts are instead abused by their rulers, the Monarchs. The EMMA application is used by the Monarchs to bait other people into accidentally intruding their Jail and have their Desires stolen. As most victims cannot defend themselves in the Metaverse, the Jail's Shadows forcefully take their Desires by plunging their arm into their chest to remove a diamond-shaped object. While the victims are not physically harmed, they will experience a Change of Heart and return to the real world mesmerized by the Monarch, causing obsessive or violent behavior among them that might result in property loss or even violent incidents. Furthermore, any opposition to the Monarch, real or perceived, can get their hearts changed with no physical use of the EMMA app. Unlike the Phantom Thieves' Change of Hearts, this is summed up as nothing short of brainwashing, so they must go into the Jails in order to confront the Monarch and induce a proper Change of Heart on them.
Since Jails do not have a treasure, other methods must be used to take the Desires back to their rightful owners. Monarchs are actually trapped inside a bird cage or a personification of their personal traumas or other disdain, preventing them from escaping and moving away from their past, and forces them to act with a "pay evil unto evil" mindset in mind. This Bird Cage must be infiltrated and its resident Lock Keeper must be destroyed.
After the Lock Keeper Shadow is destroyed and the Monarch is sent a Calling Card in the real world, their Shadow will manifest to defend their accumulated Desires. Unlike Palace owners, simply defeating them and convincing them to take off their crown will promptly release all Desires kept hostage and change the Monarch's heart, returning all back to normal.
If there is no Monarch, then the Lock Keeper also serves as an interim; its destruction will release all of the Desires held in the Jail.
The Change of Heart given by the Phantom Thieves results in the former Monarchs confessing their crimes the next day on public television. Although the Monarch does not have any memory of their Change of Heart, their mental state is the same as they were before due to the lack of a Prison of Regression or equivalent, making them capable of cooperating with the police, which is almost always the case.
While EMMA is already programmed to change hearts to grant humanity's desires, it was only able to use this function after Akira Konoe and Shuzo Ubukata modified it with the ability of cognitive psience, allowing it to generate Jails autonomously and control the Monarch's mentality to match Konoe's mindset, resulting in the Monarchs behaving exactly like whomever traumatized or wronged them as long as they are in charge of the Jails. Konoe sought to use this behavior to wipe out all crime from earth, with a severe disregard of free will.
However, usage of the Jail function by Monarchs to change the public's hearts appears to be separate from the game's final act, where EMMA itself becomes a crazed monstrosity and brainwashes masses of people into giving up their Desires. Instead, daily interaction with the application triggers this behavior from EMMA. This stems from Kuon Ichinose refusing to program EMMA with a proper emotional and understanding algorithm, resulting in EMMA not being able to collect any Desires save for people asking her for advice. Therefore, when she awakens to grant the public's wishes, she was led to believe that the public wished to have it think for them, resulting in the events of the final act.
Despite this incident shaking all of Japan within a few months, Ichinose did not commit any crimes other than abusing the supernatural. Therefore, she was set free and the Phantom Thieves were proven innocent alongside her, with only Konoe arrested for murder and bribery charges.